The expression “process performed in semi-continuous mode” means here a process in which at least one of the reagents is, from the start of the reaction, introduced in total into the photochemical reactor while at least one of the reagents is introduced gradually into the reactor as it is consumed. Such a procedure is occasionally found to be necessary in order to observe the optimum synthetic conditions.
In a photochemical process operating in liquid medium, it is necessary for the radiating portion of the lamp to be constantly fully immersed in the liquid reaction medium. This total immersion is essential so as not to illuminate a gas phase whose behaviour, under ultraviolet irradiation, may prove to be hazardous or an inconvenience for the reaction under consideration. Thus, for example, in the synthesis of sulphides and mercaptans by photochemical reaction of an alkene with a mercaptan or hydrogen sulphide, UV irradiation of the gas phase may produce elemental sulphur which is a well-known inhibitor of free-radical reactions such as those carried out to produce sulphides or mercaptans. Moreover, total immersion of the radiating portion of the lamp also makes it possible to remove, via the reaction medium, the heat emitted by the lamp; this arrangement simplifies the process and minimizes the equipment required (jacket, cooling-water circuit, exchangers, etc.).
However, this arrangement is incompatible with the use of a semi-continuous process in which the gradual introduction of at least one of the reagents entails an increase in the volume of the reaction medium in a reactor of fixed geometry.